CN115819630A

CN115819630A - Method for preparing high-charge-capacity nanocellulose

Info

Publication number: CN115819630A
Application number: CN202211728204.8A
Authority: CN
Inventors: 杨轩; 王文俊; 张达; 林嘉河
Original assignee: Quzhou Research Institute of Zhejiang University
Current assignee: Quzhou Research Institute of Zhejiang University
Priority date: 2022-12-29
Filing date: 2022-12-29
Publication date: 2023-03-21

Abstract

The invention discloses a method for preparing nano-cellulose, which comprises the following steps: (1) Dispersing a biomass raw material in binary anhydride and 0-5% of water, performing esterification pretreatment reaction at a temperature of below 120 ℃ to obtain a pretreatment reaction product, washing off redundant binary anhydride with water, and adjusting the pH of the pretreatment reaction product to be neutral to obtain a neutral pretreatment reaction product; (2) Dispersing the neutral pretreatment reaction product in water, and performing dissociation treatment on the neutral pretreatment reaction product by adopting microfibrillar equipment to obtain nano cellulose; and (3) optionally, dehydrating the washing wastewater produced in the step (1) to recover the dibasic acid anhydride therein, and using steam produced by the dehydration for supplying heat to the reactor and/or using water obtained by condensing the steam for washing.

Description

Method for preparing high-charge-capacity nanocellulose

Technical Field

The present invention relates to a method for preparing nanocellulose with high charge capacity, and nanocellulose with high charge capacity obtained by the method.

Background

The nano-cellulose (Ce l u l ose Nanofi br i l, CNF) is a cellulose material with nano-size (the diameter is less than 100 nm), and has the advantages of high mechanical strength (the tensile strength is about 1-5GPa, the elastic modulus of a crystal region is 100 GPa), high reaction activity, reproducibility, biodegradability and the like, so the nano-cellulose has wide application prospect.

CNFs with high charge have high solution stability and are more likely to form crosslinked systems due to sufficient repulsion between nanofibers, and thus have great potential in applications such as gels, thickeners, reinforcing fillers, etc.

In the prior art, methods of preparing CNFs include first introducing charged groups on the cellulose surface by a one-step pretreatment process, followed by fibrillation of the CNFs by using mechanical exfoliation (e.g., high pressure homogenization, milling, microfluidization, etc.).

The pretreatment process mainly comprises the following steps:

1) Oxidative pretreatment using 2,2,6,6-Tetramethy l p i per i d i ny l oxy (TEMPO) (see e.g. Bi omacromo l ecu l es,2006, 7 th, page 1687) in which a TEMPO mediated oxidation system is used to oxidize the primary hydroxyl groups of cellulose to produce CNF with a negatively charged surface. The CNF prepared by this process at a temperature of 25 ℃ is thin in diameter (about 5 nm), and the process is widely used in laboratories. However, problems with this process include: TEMPO is toxic and expensive, and TEMPO-containing wastewater is treated, thereby causing difficulty in popularization in practical industrial processes.

2) Acid hydrolysis pretreatment (see e.g. chem nano mat,2017, 3 rd, 328-335 p) wherein kraft pulp is treated with maleic acid aqueous solution (about 15-75%) at 120 ℃ to obtain a partly esterified CNF, the CNF prepared by this process has a low carboxyl content (about 0.14 mmol/g) and a high degree of fiber hydrolysis, the resulting CNF has a short length (about 220 nm), which seriously affects the mechanical properties of the product.

3) Anhydride pre-treatment (see e.g. Bi omacromo l ecu l es,2017, stage 18, page 242) where succinic anhydride, maleic anhydride, phthalic anhydride are used to esterify wheat fibres to give CNFs of finer diameter (about 5 nm). However, this process is carried out at a temperature of 90 ℃, requires the use of N, N-Dimethylformamide (DMF) as a solvent and pyridine as a catalyst, and the use of such volatile organic solvents and toxic catalysts increases operational risks and post-processing difficulties.

As can be seen from the above analysis, the methods for preparing CNF in the prior art have the following problems:

1) The method is not environment-friendly enough, the use of volatile organic solvents and toxic substances increases the danger in the operation process, a large amount of VOC waste water is generated, and reaction reagents are difficult to recover;

2) The reaction in the pretreatment process is severe, resulting in excessively high degree of fiber hydrolysis, and thus the prepared CNF has a short length (< 500 nm), resulting in poor mechanical properties of the product;

3) The requirements on raw materials are high, and one preparation method is generally only suitable for one raw material.

Therefore, there remains a great challenge to produce high charge and uniform-sized CNFs at mild, environmentally friendly, and low energy consumption.

Drawings

Fig. 1 is a TEM photograph of the CNF dispersion prepared in example 1.

Fig. 2 is a TEM photograph of the CNF dispersion prepared in example 2.

Fig. 3 is a TEM photograph of the CNF dispersion prepared in example 3.

Fig. 4 is a TEM photograph of the CNF dispersion prepared in example 4.

Fig. 5 is a TEM photograph of the CNF dispersion prepared in example 5.

Fig. 6 is a TEM photograph of the CNF dispersion prepared in comparative example 1.

Fig. 7 is a TEM photograph of the CNF dispersion prepared in comparative example 2.

Disclosure of Invention

The invention aims to provide a method for preparing high-charge-capacity nanocellulose under mild, environment-friendly and low energy consumption aiming at the problems in the prior art, which adopts binary anhydride as a pretreatment reagent, and makes different biomass raw materials undergo esterification reaction at a lower temperature (less than or equal to 120 ℃), so that a large amount of carboxyl groups are grafted on the surfaces of fibers, and the repulsion between the fibers in an aqueous solution is increased, therefore, the high-charge-capacity nanocellulose can be prepared under mild, environment-friendly and low energy consumption.

Thus, in one aspect, the present invention relates to a method for preparing nanocellulose, comprising the steps of:

(1) Dispersing a biomass raw material in dibasic acid anhydride and 0-5% of water, performing esterification pretreatment reaction at the temperature of below 120 ℃ to obtain a pretreatment reaction product, washing redundant dibasic acid anhydride by using water, and adjusting the pH of the pretreatment reaction product to be neutral to obtain a neutral pretreatment reaction product;

(2) Dispersing the neutral pretreatment reaction product in water, and performing dissociation treatment on the neutral pretreatment reaction product by adopting microfibrillar equipment to obtain nano cellulose; and

(3) Optionally, the washing wastewater produced in the step (1) is subjected to dehydration treatment to recover the dibasic acid anhydride therein, and the steam produced by the dehydration treatment is used for supplying heat to the reactor and/or the water obtained by condensing the steam is used for washing.

In the esterification pretreatment reaction of the above (1), the temperature to be used is critical and is controlled to be below 120 ℃, preferably about 20 to 120 ℃, more preferably about 60 to 110 ℃, most preferably about 80 to 100 ℃; the pressure conditions employed are generally about 1 to 10bar, preferably about 1 to 3bar; the pretreatment time is usually about 0.5 to 24 hours, preferably about 1 to 5 hours.

The biomass feedstock is those known in the art and typically includes, but is not limited to, wood (e.g., softwood, hardwood), grasses, crop stalks, nutshells, pine cones, plant residues (e.g., bagasse, coffee bean dregs), and the like. Preferred biomass feedstocks for use in the process of the present invention include, for example, pine, cedar, fir, eucalyptus, poplar, birch, phyllostachys pubescens, rice straw, corn stover, wheat straw, sorghum stover, reed, peanut hulls, rice hulls, pine cones, bagasse, coffee bean dregs, and the like. In the present invention, one biomass raw material or a mixture of two or more biomass raw materials may be used. As is known in the art, the biomass feedstock is typically first pulverized into a powder or granules at about 4-200 mesh via a pulverizer before being dispersed in the dibasic acid anhydride.

In some embodiments, the biomass feedstock can be pulp fibers, which are treated fiber products commonly used in the pulp and paper industry, such as softwood pulp fibers, hardwood pulp fibers, straw pulp fibers, bamboo pulp fibers, and the like, which can be unbleached products or bleached products. In the present invention, the above pulp fiber in the form of a dried pulp sheet is used. These pulp fibers in the form of dried pulp sheets are directly commercially available.

The dibasic acid anhydrides are those commonly used in the art and include, for example, malonic anhydride, maleic anhydride (maleic anhydride), dimethylmaleic anhydride, fumaric anhydride, succinic anhydride, methylsuccinic anhydride, 2-hydroxysuccinic anhydride, (2-propenyl) succinic anhydride, S-acetylmercaptosuccinic anhydride, cis-3-carboxypentenoic anhydride, trans-1, 2-cyclohexanedicarboxylic anhydride, glutaric anhydride, adipic anhydride, cis-4-cyclohexene-1, 2-dicarboxylic anhydride, cyclohexane-1, 2-dicarboxylic anhydride, pimelic anhydride, suberic anhydride, azelaic anhydride, dodecylsuccinic anhydride, dodecenylsuccinic anhydride, phthalic anhydride, 4-chlorophthalic anhydride, tetrachlorophthalic anhydride, 3-fluorophthalic anhydride, 3, 6-difluorophthalic anhydride, tetrafluorophthalic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, citric anhydride, and the like. In the process of the present invention, preferred anhydrides are selected from maleic anhydride, succinic anhydride, glutaric anhydride and tetrahydrophthalic anhydride. In the present invention, one dibasic acid anhydride or a mixture of plural dibasic acid anhydrides may be used.

The weight ratio of the biomass feedstock to the dibasic acid anhydride is typically from about 5.

In the esterification pretreatment reaction of the above (1), in addition to the dibasic acid anhydride, about 0 to 5% of water, preferably about 0 to 2% of water, based on the total weight of the biomass raw material, the dibasic acid anhydride and the water, may be added. In the process of the present invention, preferably, no water is present in the esterification pretreatment reaction, i.e., the esterification pretreatment reaction is carried out in an anhydrous system. Wherein the water includes water contained in the raw material and additionally added water, so that the content of water refers to the sum of the water contained in the raw material and the additionally added water. Thus, in the absence of water in the esterification pretreatment reaction, all of the starting materials are dry, containing no water.

In the esterification pretreatment reaction of the above (1), excess dibasic acid anhydride is washed away with water to recover and reuse the dibasic acid anhydride.

In the esterification pretreatment reaction of the above (1), the step of adjusting the pH of the pretreatment reaction product to neutrality is known in the art, and is usually carried out with an alkali solution which is an aqueous solution of an alkali known in the art, including, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogencarbonate, potassium hydrogencarbonate, sodium hydrogensulfate, potassium hydrogensulfate, sodium hydrogenphosphate, potassium monohydrogen phosphate and the like. The concentration of the alkali solution is generally about 0.1-40%.

In the dissociation step of (2) above, the microfibrillating apparatus is known in the art and includes, for example, an ultrasonic apparatus, a high-pressure homogenizer, a ball mill, a disk mill, a microfluidizer, a blender, an ice crusher, and the like. Conditions for dissociation using microfibrillating equipment are also known in the art, e.g., the temperature is typically room temperature; in the case of a blender or ice breaker, the blending speed is typically about 5000-40000rpm; in the case of a high-pressure homogenizer, the piston pitch is usually about 50 to 500 μm, and the pressure is usually about 10 to 500MPa; in the case of an ultrasonic instrument, the power is typically about 500-2000W; in the case of a microfluidizer, the diameter of the tube is generally from about 50 to 500. Mu.m, and the pressure is generally from about 10 to 800MPa.

The relative amount ratio of the neutral pretreatment reaction product to water can vary over a wide range, and is generally from about 0.01.

In the above optional step (3), the washing wastewater is subjected to dehydration treatment to remove water therefrom and recover the dibasic acid anhydride, the dehydration treatment process being known in the art and including, for example, evaporation, rectification, crystallization, drying and the like, usually at a process temperature of about 25 to 200 ℃. The recovered dibasic acid anhydride may be recycled for use in step (1), and the removed moisture in the form of steam may be recycled for supplying heat to the reactor, and the water obtained by condensing the steam may be recycled for use in the washing operation in step (1).

In the process of the present invention, the recovery of the dibasic acid anhydride can be up to about 80% or more, or even about 90% or more. Wherein the recovery of the dibasic acid anhydride is calculated by the following formula:

recovery% = dry mass of recovered dibasic acid anhydride/(mass of dibasic acid anhydride raw material-mass of dibasic acid anhydride consumed by reaction).

The nanocellulose prepared by the above method is in the form of a dispersion, which can be further dried into a powder form by a conventional drying process. The nanocellulose in powder form can be redispersed in water to give nanocellulose dispersions which remain stable without delamination after about 7 days, even about 6 months of standing.

The nanocellulose prepared by the above method has a high surface charge amount, a long length, a fine and uniform diameter. Typically, the nanocellulose has a high surface charge of about 300-3,000. Mu. Mol/g, preferably about 600-2000. Mu. Mol/g, a zeta potential of about-20 to-60 mV, preferably about-30 to-50 mV, a length of about 500-5,000nm, preferably about 1,000-5,000nm, and a diameter of about 3-20nm, preferably about 3-10 nm.

The method of the invention has the following advantages:

(1) By controlling the pretreatment temperature below 120 ℃, the reaction is milder, the requirement on reaction equipment is lower, the energy consumption is lower, the fiber degradation degree is lower, and the raw material yield is higher;

(2) Carboxyl is grafted on the surface of the fiber through esterification reaction with the dibasic acid anhydride, so that the repulsive force between the fibers is increased, and the dispersion of basic fibrils in an aqueous solution is promoted, therefore, microfibrillation can be realized under low energy consumption, and the obtained nano-cellulose has smaller and uniform diameter, longer length and high electric charge; and

(3) Through reasonable design reaction and separation flow, water is used as a washing solvent, the danger and leakage risk of the operation process are reduced, the recovery difficulty of the binary anhydride and the hydrosolvent is reduced, the danger grade requirement of operation equipment is reduced, and the discharge amount of VOC waste water is reduced.

In another aspect, the present invention relates to a nanocellulose having a high surface charge of about 300-3,000. Mu. Mol/g, preferably about 600-2000. Mu. Mol/g, a zeta potential of about-20 to-60 mV, preferably about-30 to-50 mV, a length of about 500-5,000nm, preferably about 1,000-5,000nm, and a diameter of about 3-20nm, preferably about 3-10 nm.

Nanocellulose in the form of the above-described dispersion can be used in those applications in which nanocellulose is commonly used, in particular in the preparation of films, wet-type auxiliaries for papermaking, gel stocks, coatings and the like.

The powder of nanocellulose in powder form described above may be used as a filler in polymer composites to improve the properties of the composites including, for example, mechanical properties, uv resistance, barrier properties, and degradation properties.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The raw materials and equipment used in the examples are all commercially available conventional products unless otherwise specified. Unless otherwise stated, the temperature and pressure conditions used refer to normal temperature and normal pressure.

The test methods employed in the examples are as follows.

The surface charge of the nanocellulose was determined by conductometry. Specifically, the ion exchange resin is used for sufficient exchange so that all the carboxyl groups on the surface of the nanocellulose exist in the form of-COOH, then 0.1 mol/L NaOH standard solution is used for titrating the CNF dispersion liquid with known concentration, and the conductivity change in the titration process is continuously measured by a conductivity measuring instrument, and the titration end point is determined according to the slope mutation.

The zeta potential of the nanocellulose was determined by dynamic light scattering.

The length and diameter of the nanocellulose were determined by using a Transmission Electron Microscope (TEM) or an Atomic Force Microscope (AFM).

Example 1: preparation of Nanocellulose (CNF) and films thereof

5g of sulfate softwood pulp fiber pulp board is ground into powder by a 100-mesh grinder, then the powder is added into 100g of maleic anhydride, and esterification pretreatment reaction is carried out for 3 hours at 90 ℃ and 1 bar; excess maleic anhydride was washed off with deionized water and the pH was adjusted to neutrality with 0.1 mol/L sodium hydroxide solution to give a neutral pretreated reaction product. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 10 minutes at 10000rpm with a high-speed stirrer to obtain a dispersion of CNF. And (4) removing water in the washing wastewater through evaporation, and recovering to obtain maleic anhydride and water.

The morphology of CNF stained with phosphotungstic acid was observed under a Transmission Electron Microscope (TEM), see figure 1.

The dispersion of the above CNF was diluted with water to a concentration of 0.05%, then formed into a film by suction filtration, and the film was dried by hot pressing at 60 ℃ for 15 minutes to give a CNF film having a dry film thickness of 30 μm.

Furthermore, the CNF dispersion described above was freeze-dried at-40 ℃ to give freeze-dried CNF, and 1 gram of the freeze-dried CNF was redispersed in 199 grams of water to give a CNF dispersion with a solid content of 0.5% by weight, which remained stable and did not delaminate after standing for 7 days.

Example 2: preparation of CNF and films thereof

Adding 5g of pine cellulose powder subjected to lignin removal by oxyacetic acid into 100g of maleic anhydride, and carrying out esterification pretreatment reaction at 95 ℃ and 1bar for 3 hours; excess maleic anhydride was washed off with deionized water and the pH was adjusted to neutrality with 0.1 mol/L sodium hydroxide solution. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 10 minutes at 10000rpm using a high-speed stirrer, to obtain a CNF dispersion. And (4) removing water in the washing wastewater through evaporation, and recovering to obtain maleic anhydride and water.

The morphology of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see figure 2.

The dispersion of CNF was diluted with water to a concentration of 0.05%, then formed into a film by suction filtration, and the film was dried by hot pressing at 60 ℃ for 15 minutes to give a CNF film having a dry film thickness of 30 μm.

In addition, the CNF dispersion was freeze-dried at-40 ℃ to give a freeze-dried CNF, and 1 g of the freeze-dried CNF was redispersed in 199 g of water to give a CNF dispersion with a solid content of 0.5% by weight, which remained stable and did not delaminate after standing for 7 days.

Example 3: preparation of CNF and films thereof

5g of dried bamboo powder was added to a mixture of 25 g of maleic anhydride and 0.5 g of water, a pretreatment esterification reaction was carried out at 95 ℃ and 1bar for 3 hours, excess maleic anhydride was washed off with deionized water, and the pH was adjusted to neutrality with 0.1 mol/L sodium hydroxide solution. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 5 minutes at 8000rpm using a high-speed stirrer, to obtain a CNF dispersion. And (4) removing water in the washing wastewater through evaporation, and recovering to obtain maleic anhydride and water.

The morphology of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see figure 3.

The above CNF dispersion was diluted with water to a concentration of 0.05%, then formed into a film by suction filtration, and the film was dried by hot pressing at 60 ℃ for 15 minutes to give a CNF film having a dry film thickness of 30 μm.

Example 4: preparation of CNF and films thereof

5g of kraft softwood pulp fiber pulp board was ground by a 100 mesh pulverizer and then added to 100g of succinic anhydride, an esterification pretreatment reaction was carried out at 118 ℃ and 2bar for 3 hours, then excess succinic anhydride was washed away with deionized water, and the pH was adjusted to neutral with 0.1 mol/L sodium hydroxide solution. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 10 minutes at 10000rpm using a high-speed stirrer to obtain a CNF dispersion, and water in the washing wastewater was removed by evaporation, and succinic anhydride and water were recovered.

The appearance of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see figure 4.

Example 5: preparation of CNF and films thereof

5g of a kraft softwood pulp sheet were milled by a 100 mesh mill and then added to 100g of tetrahydrophthalic anhydride and 1 g of water, and an esterification pretreatment reaction was carried out at 110 ℃ and 2bar for 3 hours, then excess tetrahydrophthalic anhydride was washed away with deionized water, and the pH was adjusted to neutrality with 0.1 mol/L sodium hydroxide solution. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 10 minutes at 10000rpm with a high-speed stirrer to obtain a CNF dispersion. And removing water in the washing wastewater through evaporation, and recovering to obtain tetrahydrophthalic anhydride and water.

The morphology of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see figure 5.

The CNF dispersion was diluted with water to a concentration of 0.05%, then formed into a film by suction filtration, and the film was dried by hot pressing at 60 ℃ for 15 minutes to obtain an LCNF film having a dry film thickness of 30 μm.

Example 6: preparation of CNF and films thereof

5g of sulfate softwood pulp fiber pulp board is ground into powder by a 100-mesh grinder, then the powder is added into 100g of maleic anhydride, 1 g of water is added, and esterification pretreatment reaction is carried out for 3 hours at 90 ℃ and 1 bar; excess maleic anhydride was washed off with deionized water and the pH was adjusted to neutrality with 0.1 mol/L sodium hydroxide solution to give a neutral pretreated reaction product. Then, the neutral pretreatment product was dispersed in water, and dissociation treatment was performed for 10 minutes at 10000rpm with a high-speed stirrer to obtain a CNF dispersion. And (4) removing water in the washing wastewater through evaporation, and recovering to obtain maleic anhydride and water.

The above CNF dispersion was diluted with water to a concentration of 0.05%, then formed into a thin film by suction filtration, and the thin film was dried by hot-pressing at 60 ℃ for 15 minutes to obtain an LCNF thin film having a dry film thickness of 30 μm.

Comparative example 1: preparation of control CNF and films thereof

Example 1 was repeated except that 6 g of water was added in the esterification pretreatment reaction.

The morphology of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see figure 6.

Comparative example 2: preparation of control CNF and films thereof

Example 1 was repeated except that the esterification pretreatment reaction was carried out at 130 ℃.

The morphology of CNF stained with phosphotungstic acid was observed under a transmission electron microscope, see fig. 7.

Example 7 testing of CNF Performance

In this example, the surface charge amount, zeta potential, average length, average diameter, and recovery rate of the dibasic acid anhydride of the CNF dispersions prepared in examples 1 to 6 and comparative examples 1 to 2, respectively, were tested by the above-described methods. The results are shown in table 1 below.

TABLE 1

The above results show that:

(1) According to the invention, carboxyl is grafted on the surfaces of different plant fiber raw materials by using esterification reaction with dibasic acid anhydride, so that the surface charge quantity of CNF is increased, the repulsive force between fibers is increased, and the dispersion of basic fibrils in an aqueous solution is promoted, therefore, microfibrillation can be realized under low energy consumption, and the obtained nanocellulose has the advantages of smaller and uniform diameter, longer length and high charge quantity;

(2) By reasonably designing the reaction and separation processes and using water as a washing solvent, the dangerousness and leakage risk in the operation process are reduced, the effective recovery of anhydride is realized, and the discharge amount of VOC waste water is reduced;

(3) By controlling the water content of the system to be below 5%, the esterification reaction degree is higher, and CNF with smaller diameter and higher charge can be obtained; and

(4) When the same dibasic acid anhydride is used for esterification pretreatment reaction, the esterification pretreatment temperature is controlled below 120 ℃, so that the reaction is milder, the fiber degradation degree is lower, and the prepared CNF has longer length.

Example 8: testing the Performance of CNF

In this example, the mechanical properties of the CNF films prepared in examples 1 to 6 and comparative examples 1 to 2 were tested, respectively.

The mechanical properties of the films were measured using a model 3345 tensile tester (I nstron, USA) with reference to GB/T1040.3-2006. The actual thickness of the test specimens was measured with a CT-100 type thickness gauge, at least 7 specimens were prepared for each group, and the average value was taken, and the tensile rate of the test was 2.5mm/mi n.

The test results are shown in table 2 below.

TABLE 2

Sample (I)	Tensile strength (MPa)	Elongation at Break (%)	Young's modulus (GPa)	Toughness (MJ/m) ³ )
					Example 1-1	200±10	12.2±0.4	8.7±0.7	16.4±0.9
Examples 1 to 2	274±6	9.5±0.7	10.7±0.5	16.8±0.9
					Examples 1 to 3	294±6	5.2±0.4	13.1±0.9	9.9±0.7
Examples 1 to 4	195±4	6.3±0.5	11.5±0.4	8.1±1.5
					Examples 1 to 5	180±9	15.0±0.6	6.3±0.6	17.3±0.9
Examples 1 to 6	189±13	11.8±0.4	10.5±0.7	15.2±1.4
					Comparative examples 1 to 1	121±9	3.6±0.2	8.9±0.3	3.2±0.7
Comparative examples 1 to 2	101±10	1.6±0.2	9.2±0.5	1.0±0.5

The results show that the CNF film prepared by the method has better mechanical property due to the characteristics of small and uniform diameter, longer length, high fiber degradation degree and the like.

The above embodiments are only specific examples of the present invention, but the scope of the present invention is not limited thereto, and any modifications, equivalents and improvements made by those skilled in the art within the technical scope of the present invention as disclosed in the present invention should be covered within the scope of the present invention.

Claims

1. A method of preparing nanocellulose, comprising the steps of:

(1) Dispersing a biomass raw material in binary acid anhydride and 0-5%, preferably 0-2% of water, performing esterification pretreatment reaction at a temperature of below 120 ℃ to obtain a pretreatment reaction product, washing off redundant binary acid anhydride with water, and adjusting the pH of the pretreatment reaction product to be neutral to obtain a neutral pretreatment reaction product;

2. The process according to claim 1, wherein the temperature used in the esterification pretreatment reaction of (1) is 20 to 120 ℃, preferably 60 to 110 ℃, more preferably 80 to 100 ℃.

3. The method of claim 1, wherein the biomass feedstock is selected from the group consisting of wood, grasses, crop stalks, husks, pine cones, and other plant residues.

4. The method of claim 3, wherein the biomass feedstock is selected from pine, cedar, fir, eucalyptus, poplar, birch, phyllostachys pubescens, rice straw, corn stover, wheat straw, sorghum straw, reed, peanut hulls, rice hulls, pine cones, bagasse, and coffee bean dregs.

5. The method of claim 1 wherein the biomass feedstock is selected from softwood pulp fibers, hardwood pulp fibers, straw pulp fibers, and bamboo pulp fibers.

6. The method of any one of claims 1-5, wherein the dibasic acid anhydride is selected from the group consisting of malonic anhydride, maleic anhydride, dimethyl maleic anhydride, fumaric anhydride, succinic anhydride, methyl succinic anhydride, 2-hydroxysuccinic anhydride, (2-propenyl) succinic anhydride, S-acetylmercaptosuccinic anhydride, cis-3-carboxypentenoic anhydride, trans-1, 2-cyclohexanedicarboxylic anhydride, glutaric anhydride, adipic anhydride, cis-4-cyclohexene-1, 2-dicarboxylic anhydride, cyclohexane-1, 2-dicarboxylic anhydride, pimelic anhydride, suberic anhydride, azelaic anhydride, dodecyl succinic anhydride, dodecenyl succinic anhydride, phthalic anhydride, 4-chlorophthalic anhydride, tetrachlorophthalic anhydride, 3-fluorophthalic anhydride, 3, 6-difluorophthalic anhydride, tetrafluorophthalic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, and citric anhydride.

7. The method of claim 6, wherein the dibasic acid anhydride is selected from the group consisting of maleic anhydride, succinic anhydride, glutaric anhydride, and phthalic anhydride.

8. The process according to any one of claims 1 to 5, wherein the weight ratio of the biomass feedstock to the dibasic acid anhydride is from 5.

9. The process of any one of claims 1-5, wherein water is not present in the esterification pretreatment reaction of (1), i.e., the esterification pretreatment reaction is conducted in an anhydrous system.

10. Nanocellulose obtainable by the process of any one of claims 1 to 9.

11. The nanocellulose of claim 10, having a high surface charge of 300-3,000 μmol/g, preferably 600-2,000 μmol/g, -a zeta potential of 20 to-60 mV, preferably-30 to-50 mV, a length of 500-5,000nm, preferably 1,000-5,000nm, and a diameter of 3-20nm, preferably 3-10 nm.